The Effect of Daylighting Strategies on Building Cooling Loads and Overall Energy Performance

It has been demonstrated that daylighting, by reducing electric lighting requirements, is one of the most effective energy-conservation strategies in office building envelope design. Additionally, because the luminous efficacy of daylight outdoors is higher than that of most electric lighting systems, it is frequently assumed that buildings designed for daylighting will have smaller cooling loads than similar buildings not designed for daylighting. This assumption is valid only within certain specific design limits. Outside these limits, daylighting may increase cooling loads, requiring larger chillers and associated cooling equipment,sand may seriously compromise or even negate the economic benefits of reduced electric lighting use. In this paper we discuss these limits, the luminous efficacy of delivered daylight in sidelighted and toplighted spaces, methods of enhancing efficacy, and the resultant overall energy and economic impacts of daylighting design.

Using DOE-2.1C as the building simulation tool, our sensitivity studies examine the cooling load effects of daylighting as a function of the following design strategies: optical properties of fenestration; spatial distribution of luminous flux; use of fixed and operable solar control parameters; and electric lighting power density, flux distribution, and control systems. We identify combinations of these parameters that maximize daylighting efficacy and discuss their effects on net annual energy consumption, peak electrical demand, and chiller size. The energy performance of fenestration can be enhanced by using advanced optical materials that increase the efficacy of daylighting by selectively controlling daylight and solar transmission. We discuss the performance of these materials, which, although technically feasible, are not yet commercially available.